Slope stability is a critical safety and production issue for coal mines. Major wall failures can occur seemingly without warning, causing loss of lives, damage to equipment and disruption to the mining process. US Mining Safety and Health Administration reports indicate that highwall fatalities account for around 10% of surface fatalities in US coal mines.
Tell-tale signs of slope instability include the opening of cracks on the wall surface and crest, audible creaking, and increased rilling of spoil. It is difficult to predict the progression of such signs to slope instability. With highwalls and in-pit benches in particular, movements may accelerate with little or no warning. Hence, mines take a conservative approach when deciding whether to expose personnel and equipment near a potentially unstable slope. Over-cautious decisions impact on the mine's productivity.
A more reliable determinant of slope instability is the measurement of outward movement and acceleration of material as an instability mechanism develops. There is strong evidence that small precursor movements of the rock wall occur for an extended period (weeks to months) prior to collapsing [E. Hoek and J. W. Bray, Rock Slope Engineering, The Institute of Mining and Metallurgy, 1981]. The acceleration of movement and the point of failure vary for different slope failure mechanisms. The type of failure mechanism depends on the rock mass and underlying geology. For example, wedge and plane-type failures occur where highly faulted/jointed rock masses and steeply dipping coal seams are prevalent. Another example is circular-type failures of low-wall spoil piles, where the adjacent coal recovery destabilises the pit floor below the spoil pile.
There are various monitoring systems that can measure the movement of the rock face. Geotechnical specialists can interpret the pattern and history of movement to improve prediction of the failure process, and to advise appropriate and timely stabilisation or safety management actions. Mine Managers can use such information to more reliably assess risk and maintain records for due diligence purposes. In addition, monitoring systems can sound an alarm to warn workers when the movement or acceleration of the rock face exceeds a set threshold.
Current monitoring methods at coal mines use extensometers and laser EDM (Electronic Distance Measurement) to measure the dilation of cracks appearing on the crest or face of the rock slope. These methods monitor points or lines on the wall rather than the area of the wall face, which makes interpretation of failure mechanisms very difficult [T. D. Sullivan, “Understanding pit slope movements”, Geotechnical Instrumentation and Monitoring in Open Pit and Underground Mining, pp, 435-445, June 1993]. In addition, these methods are costly and time consuming to set up and relocate because they usually require the careful placement of sensors or reference reflectors on unstable ground that is potentially dangerous and difficult to access. Other researchers have suggested the use of reflector-less laser EDM, however currently available systems are extremely expensive to purchase, and they do not have the desired accuracy (errors of ±5 to ±10 mm when measuring to a perpendicular flat surface [Leica, WILD DI3000S Series Brochure, Leica AG, Switzerland.]). The accuracy is significantly worse for darker and rougher surfaces that are more characteristic of rock walls. The accuracy also worsens significantly for ranges greater than 50 meters.